In modern residential flats and other indoor spaces, especially those which are centrally heated, the air is often too dry. Therefore, endeavours have been made to increase artificially the relative humidity of the room air. Small, electrically operated air humidifiers which are kept on the floor have become popular, in which the vaporizer heats water, converting it into water vapour which, being lighter than air, spreads into the room air from the apparatus.
In practice, air humidifiers of the prior art have however proved inconvenient, and they require a lot of maintenance. The user is most strongly inconvenienced by the water tank of the apparatus, which has to be filled at regular intervals. In addition, the air humidifier must absolutely be cleaned thoroughly fairly often because in use impurities dangerous to health tend to accumulate in the vaporizer and are spread by the apparatus into the room air. The humidity of the ambient air is also overlooked by existing air humidifiers, which instead humidify the room air at a given constant rate all the time.